A novel type D retrovirus was isolated by cocultivation of explants of fibromatous tissue from a rhesus monkey (Macaca mulatta) with immunodeficiency and retroperitoneal fibromatosis (RF). This type D virus, isolated from a macaque with simian acquired immunodeficiency syndrome (SAIDS-D/Washington), is exogenous and is partially related to the Mason-Pfizer and the langur monkey type D viruses. The SAIDS-D/Washington virus can be distinguished from all other primate retroviruses by antigenicity and molecular hybridization. Nucleic acid hybridization studies reveal that the origin of the SAIDS-D isolate may reside in Old World monkey (subfamily Colobinae) cellular DNA. Foci which suggest a transformed phenotype have been noted after SAIDS-D/Washington infection of some continuous cell lines. A biochemical and immunological characterization of SAIDS-D/Washington viral proteins is in progress. The SDS-PAGE profiles of polypeptides among type D retroviruses (MPMV, langur endogenous virus, SAIDS-D/Washington) from Old World monkeys are similar, but differences become evident upon HPLC separation of viral polypeptides and subsequent amino acid sequence analysis. By radioimmunoassay of p10 and glycoprotein antigens, the SAIDS-D/Washington retroviral isolate from RF tissue can be distinguished from two type D viruses isolated at the California and New England Regional Primate Research Centers from blood of monkeys with SAIDS. Although some inoculated macaques are viremic, and control macaques are not, no solid tumors of RF, nor immunologic abnormalities diagnostic of SAIDS, have been observed in a cell-free transmission study of eight M. nemestrina after three months post-inoculation of SAIDS-D/Washington virus.